Solar photovoltaic systems

ABSTRACT

We describe a photovoltaic (PV) power generation system comprising at least two PV panels and a power conditioning unit. The dc power outputs of the PV panels are connected in parallel to a dc power input of the power conditioning unit. The power conditioning unit comprises a dc-to-dc converter having an input coupled to the dc power input and an output coupled to a dc link of the unit, a dc-to-ac converter having an input coupled to the dc link and an ac mains power supply output, and an energy storage capacitor coupled to the dc link. The power conditioning unit is configured to perform maximum power point tracking (MPPT) responsive to a level of power flowing into the dc power input, and the level of power flowing into said dc power input is sensed at the dc link. In preferred implementations the energy storage capacitor is a non-electrolytic capacitor.

RELATED APPLICATIONS

The present invention claims priority from British Application No.GB1009430.8, filed 7 Jun. 2010, incorporated herein in its entirety.

FIELD OF THE INVENTION

This invention relates to improved techniques for photovoltaic powergeneration with maximum power point tracking (MPPT).

BACKGROUND TO THE INVENTION

Background prior art relating to solar inverters and MPPT in general canbe found in: EP0780750A; JP2000020150A; US2005/0068012; JP05003678A;GB2415841A; EP0947905A; WO2006/011071; EP1,235,339A; WO2004/006342;DE100 64 039A; US2005/030772; WO96/07130; U.S. Pat. No. 6,657,419;US2004/117676; US2006/232220; WO2004/001942; GB2419968A; U.S. Pat. No.7,319,313; U.S. Pat. No. 7,450,401; U.S. Pat. No. 7,414,870; U.S. Pat.No. 7,064,967; “Cost-Effective Hundred-Year Life for Single-PhaseInverters and Rectifiers in Solar and LED Lighting Applications Based onMinimum Capacitance Requirements and a Ripple Power Port”, P. T. Kerinand R. S. Balog—technical paper; US2009/0097283; “Long-Lifetime PowerInverter for Photovoltaic AC Modules”, C. Rodriguez and G. A. J.Amaratunga, IEEE Trans IE, 55(7), 2008, p 2593; and US2008/097655.

We have previously described improved techniques for maximum power pointtracking (MPPT) for solar invertors (see our UK patent application No.1004621.7 filed 19 Mar. 2010 and U.S. patent Ser. No. 12/789,154 filed27 May 2010). It has been recognised that these techniques facilitatethe viable use of novel solar photovoltaic system architectures.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provideda photovoltaic power generation system, the system comprising: at leasttwo photovoltaic panels each having a dc power output; a powerconditioning unit having a dc power input and an ac mains power supplyoutput for delivering an ac mains supply; wherein said dc power outputsof said at least two photovoltaic panels are connected in parallel withone another to said dc power input of said power conditioning unit;wherein said power conditioning unit comprises a dc-to-dc converterhaving an input coupled to said dc power input and having an outputcoupled to a dc link of said power conditioning unit, a dc-to-acconverter having an input coupled to said dc link and having an outputcoupled to said ac mains power supply output, and an energy storagecapacitor coupled to said dc link; wherein said power conditioning unitis configured to perform maximum power point tracking (MPPT) responsiveto a level of power flowing into said dc power input, and wherein saidlevel of power flowing into said dc power input is sensed at said dclink.

In a related aspect the invention provides a photovoltaic powergeneration system, the system comprising: at least two photovoltaicpanels each having a dc power output; a power conditioning unit having adc power input and an ac mains power supply output for delivering an acmains power supply; wherein said dc power outputs of said at least twophotovoltaic panels are connected in parallel with one another to saiddc power input of said power conditioning unit; wherein said powerconditioning unit includes an energy storage capacitor to store energyfrom said photovoltaic panels and a dc-to-ac converter having a dc inputcoupled to said energy storage capacitor and an ac output coupled tosaid ac mains power supply output; wherein said power conditioning unitcomprises a controller coupled to control said dc-to-ac converter toperform maximum power point tracking (MPPT); wherein said controller hasa sense input coupled to said energy storage capacitor to sense at saidenergy storage capacitor a signal responsive to a level of power flowinginto said dc power input of said power conditioning unit, wherein saidcontroller is configured to perform said MPPT by controlling saiddc-to-ac converter to control power injected into said ac mains powersupply responsive to said level of power flowing into said dc input ofsaid power conditioning unit sensed at said energy storage capacitor.

Broadly speaking, the inventors have recognised that by using an energystorage capacitor on the dc link, and by performing maximum power pointtracking (MPPT) based upon a power flow sensed at this link two or moreparallel connected (that is positive to positive and negative tonegative connected) solar photovoltaic panels may be employed and stillachieve almost an optimal harvesting of power from the pair (or more) ofpanels: Experiments have shown that the difference in performancebetween separate MPPT for each panel and combined MPPT trackingaccording to embodiments of the invention for a pair of parallelconnected panels is typically only of order 1%, even where shadingeffects and the like are present.

Further benefits of embodiments of the invention are that theapproximate ac mains power generation cost per watt is approximatelyhalved, since only a relatively small up-rating of the inverter istypically needed, for example by increasing the value of the energystorage capacitor by perhaps 30%, up to say 36 μF for an ac power outputof up to around 350 watts. Moreover in general the power conditioningunit (inverter) operates more efficiently with greater input power, inpart because of a fixed overhead for the power required by the internalcircuitry (which is particularly significant for microinverters).Because of this providing an input from two or more parallel connectedpanels tends to maintain the input power even under cloudy conditions,maintaining the inverter in a more efficient mode of operation for one.

In some preferred embodiments the power conditioning unit tracks theMPPT without directly measuring a dc voltage or current from thepanels—instead in embodiments a voltage (but not a current) is sensed onthe dc link to which the energy storage capacitor is connected. In somepreferred embodiments, as described in more detail later, a controllersenses the level of a ripple voltage on this link to sense anintermediate power flow through the dc link which, in the absence oflosses, measures a combined dc input power flow from the photovoltaicpanels to the power conditioning unit. In embodiments the controllercontrols the dc-to-ac converter to maximise the ripple voltage and hencethis intermediate power flow, thereby maximizing the combined dc inputpower from the pair of solar photovoltaic panels. In preferredembodiments a substantially fixed amplitude dc-to-dc converter isconnected between the dc input of the power conditioning unit and the dclink to provide a substantially fixed amplification factor increasingthe input dc voltage from the panels to an intermediate dc voltagetypically greater than 100 volts, 200 volts, 300 volts, 400 volts or 500volts.

For certain architectures there can be a benefit to employing two ormore parallel connected panels even when MPPT is not performed bysensing at the dc link as described above. This is especially so for amicroinverter, that is an inverter dedicated to one or a few PV panels.

Thus in a further aspect of the invention there is provided aphotovoltaic power generation system, the system comprising: at leasttwo photovoltaic panels each having a dc power output; a powerconditioning unit having a dc power input and an ac mains power supplyoutput for delivering an ac mains supply; wherein said dc power outputsof said at least two photovoltaic panels are connected in parallel withone another to said dc power input of said power conditioning unit;wherein said power conditioning unit comprises a dc-to-ac converter anda dc link between said dc power input of said power conditioning unitand an input of said dc-to-ac converter, wherein said dc-to-ac converterhas an output coupled to said ac mains power supply output, and whereinsaid power conditioning unit further comprises an energy storagecapacitor coupled to said dc link; wherein said energy storage capacitoris a non-electrolytic capacitor; and wherein said power conditioningunit is configured to perform maximum power point tracking (MPPT)responsive to a level of power flowing into said dc power input.

Embodiments of the above power conditioning system can provide aneffective cost saving per watt of power generated because of the way thecomponent values scale with power. More particularly because the energystorage capacitor is located at the dc link, a relatively small,non-electrolytic capacitor may still be employed (see also ourWO2007/080429, hereby incorporated by reference). With an energy storagecapacitor located at the dc link, the required energy storage is stillrelatively low even when two or more PV panels are connected inparallel. Furthermore, even where the MPPT is sub-optimal because amicroinverter can be physically located close to the PV panels to whichit is connected the voltage drop across the connecting cables (which canbe significant) is reduced, and this can help to mitigate any deficit inthe MPPT.

Some preferred embodiments of the above system employ a controller tocontrol an amplitude of an ac current injected into the ac mains suchthat it is substantially linearly dependent on or proportional to anamplitude of a sinusoidal component of ripple voltage (at twice themains frequency) on the energy storage capacitor. More particularly theac current injected into the mains is controlled by controlling thedc-to-ac converter, and in some preferred embodiments the samecontroller performs MPPT, controlling the injected current by sensing avoltage on the energy storage capacitor. Potentially a transformerlesspower conditioning unit (inverter) may be employed, but in preferredembodiments the power conditioning unit includes a dc-to-dc converter atthe front end, as previously described. For embodiments which controlthe current injected into the ac mains based on sensing a level ofripple voltage on the dc link, use of a dc-to-dc converter between thedc input of the power conditioning unit and the dc link provides aconvenient way of allowing a ripple voltage to be present on the energystorage capacitor when it is not present at the dc input of the powerconditioning unit.

In a related aspect the invention provides a method generating an acmains power supply from a plurality of photovoltaic panels, the methodcomprising: connecting dc power outputs from said photovoltaic panels inparallel to the input of a power conditioning unit; converting said flowof dc power into said ac mains power supply using said powerconditioning unit, wherein said converting comprises converting saidinput flow of dc power units into an intermediate flow of dc power on adc link of said power conditioning unit coupled to an energy storagecapacitor, and converting said intermediate flow of dc power to said acmains power supply; and tracking substantially a maximum power point ofsaid common input flow of dc power.

As previously described, in embodiments by sensing the intermediatepower flow rather than by employing MPPT tracking at the front end ofthe power conditioning unit embodiments of the invention substantiallymaximize the combined dc input power flow from the pair (or more) ofparallel-connected solar photovoltaic panels.

The invention also provides a system for generating an ac mains powersupply from a plurality of photovoltaic panels, the system comprising:means for connecting dc power outputs from said photovoltaic panels inparallel to the input of a power conditioning unit to provide a commonflow of dc power; means for converting said flow of dc power into saidac mains power supply using said power conditioning unit, wherein saidconverting comprises converting said input flow of dc power into anintermediate flow of dc power on a dc link of said power conditioningunit coupled to an energy storage capacitor; means for converting saidintermediate flow of dc power to said ac mains power supply; and meansfor tracking substantially a maximum power point of said common inputflow of dc power.

In embodiments of the above described systems preferably thephotovoltaic panels are directly connected to one another in parallelthat is without an intermediate series-connected panel or panels. Theparallel connections of the panels may be internal or external to thepower conditioning unit. However potentially, albeit less preferably,two pairs of photovoltaic panels may be connected in series and then thepairs of panels connected in parallel. However this is less preferablebecause the MPPT fails to operate properly if one panel is significantlyshaded or fails, resulting in a voltage drift.

As previously mentioned, embodiments of the techniques we describe areparticularly suitable for so-called microinverters. A microinverter maybe defined as an inverter having a power rating suitable for connectionto less than 10 or less than 5 panels and/or as an inverter having a dcinput voltage which is less than half a peak-to-peak voltage of the acmains, more typically less than 100 volts dc or less than 60 volts dc.

Preferred embodiments of the system provide single phase ac, but thetechniques we describe are not limited to use with a single phase acmains supply, and may also be applied to a photovoltaic power generationsystem providing a three phase ac mains supply. In this latter case,preferably one dc-to-ac converter per phase is employed.

Maximum Power Point Tracking

To aid in understanding the operation of embodiments of the invention wewill now describe, first in broad terms and later in detailed terms, aparticularly preferred implementation of MPPT for use with the abovedescribed solar photovoltaic system architecture.

Thus in broad terms a particularly preferred power conditioning unitwith maximum power point tracking (MPPT), for delivering power from a dcpower source to an ac mains power supply output, comprises: an input forreceiving power from said dc power source; an output for delivering acpower to said ac mains power supply; an energy storage capacitor forstoring energy from said dc power source for delivering to said ac mainspower supply output; a dc-to-ac converter coupled to said output forconverting energy stored in said energy storage capacitor to ac powerfor said ac mains power supply output; a power injection control blockhaving a sense input coupled to said energy storage capacitor and havingan output coupled to said dc-to-ac converter, to control said dc-to-acconverter to control power injected into said ac mains power supply; andwherein said power injection control block is configured to track amaximum power point of said dc power source without measuring a dcvoltage or dc current provided from said dc power source.

In embodiments a voltage on the energy storage capacitor has asinusoidal voltage component (at twice the frequency of the ac mains),and the power injection control block is configured to control anamplitude of an ac current provided to the ac mains power supply outputsuch that an amount of power transferred to the output is dependent onan amplitude of the sinusoidal voltage component on the energy storagecapacitor. In embodiments the average energy transferred is linearlydependent on, more particularly proportional to, a squared value of thesinusoidal voltage component. In embodiments the sinusoidal voltagecomponent is superimposed on a dc link voltage (input to the dc-to-acconverter), and this link voltage is relatively high, for example lessthan 200, 300, 400 or 500 volts. In such an embodiment the average powertransferred is proportional to the difference between the peak (maximum)capacitor voltage squared and the trough (minimum) capacitor voltagesquared (although alternatively a power conditioning unit may bearranged such that there is, on average, zero dc voltage on the energystorage capacitor). In embodiments the instantaneous power transferredto the ac mains power supply output is dependent on or proportional tothe instantaneous value of voltage on the energy storage capacitor.

Thus we also describe a power conditioning unit with maximum power pointtracking (MPPT), for delivering power from a dc power source to an acmains power supply output, the power conditioning unit comprising: aninput for receiving power from said dc power source; an output fordelivering ac power to said ac mains power supply; an energy storagecapacitor for storing energy from said dc power source for delivering tosaid ac mains power supply output; a dc-to-ac converter coupled to saidoutput for converting energy stored in said energy storage capacitor toac power for said ac mains power supply output; a power injectioncontrol block having a sense input coupled to said energy storagecapacitor and having an output coupled to said dc-to-ac converter, tocontrol said dc-to-ac converter to control power injected into said acmains power supply; and wherein, in operation, a voltage on said energystorage capacitor has a sinusoidal voltage component at twice afrequency of said ac mains; wherein said power injection control blockis configured for controlling an amplitude of an ac current provided tosaid ac mains power supply output such that an amount of powertransferred to said ac mains power supply output is dependent on anamplitude of said sinusoidal voltage component on said energy storagecapacitor, and wherein said power injection control block is configuredto track a maximum power point of said dc power source by controllingsaid dc-to-ac converter.

In the above described power conditioning units an energy flow from thedc power source to the energy storage capacitor is substantiallyproportional to an amount of energy change in the energy storagecapacitor (this is explained further below). Further, an amount ofenergy drawn from the energy storage capacitor and provided to the acmains output is controlled by the power injection control block suchthat the amount of ac power delivered to the ac mains power supply isdependent on the amount of energy stored in the energy storagecapacitor. In such an arrangement the power arrangement control block isthereby able to track the maximum power point of the dc power source bycontrolling the ac power delivered to the AC mains power supply bycontrolling the dc-to-ac converter, without the need for MPP tracking onthe front end of the power conditioning unit, which typically includes adc-to-dc converter. In broad terms the power injection loop pulls power,in the first instance, from the dc power source and delivers this intothe energy storage capacitor. In the second instance the power injectionloop extracts power from the energy storage capacitor and delivers thisto the AC output. The need to deliver AC power to the output results ina sinusoidal voltage component on the energy storage capacitor, and thisis an intrinsic part of this control loop; typically this fluctuatingsinusoidal component of (a generally dc) voltage on the energy storagecapacitor has, in operation, a peak amplitude of at least 10 Volts, 20Volts, 30 Volts, 40 Volts, 50 Volts, 60 Volts or 100 Volts. The peakamplitude of this sinusoidal voltage component depends upon the currentinjected into the ac mains output.

Were MPPT to be implemented at the dc input end of the powerconditioning unit, for example by means of a control loop on a front enddc-to-dc converter, an MPPT tracking algorithm would generally impose adegree of ripple on the dc input voltage to the power conditioning unit,in order that the operating point of the dc power source can be variedto hence determine the maximum power operating point. The operatingpoint automatically adjusts according to the energy change in the energystorage capacitor.

By contrast in embodiments we employ a “pull” arrangement in which powerflows from the dc power source into the energy storage capacitor ineffect on demand, the demand being controlled by the second, powerinjection control loop.

In more detail, the degree of ripple on the DC link, more particularlythe ripple amplitude, is effectively a measure of the amount of powerdrawn from the DC input, for example a solar photovoltaic panel. If theripple reduces this implies that less power is being provided from theDC input and in broad terms the power injection control block thenresponds by reducing the current injected into the grid, that is byadjusting the power injection. In embodiments the current is regulatedby adjusting the switching speed (rate) of the output DC-to-ACconverter. When the system is tracking the maximum power point, if thepower from the DC input reduces, the ripple reduces and the switchingspeed of the converter is adjusted downwards, to inject less currentinto the grid. This brings the operating point back towards the maximumpower point and balances the amount of power provided by the DC sourcewith that being injected into the grid. The control block thenperiodically increases the switching speed of the power injection blockwith the aim of increasing the amount of current flowing into the grid.This has the effect of increasing the ripple in the event that theamount of energy being provided by the DC source is greater than thatbeing harvested, and hence the control loop effectively operates so asto maximise the ripple and therefore harvested energy. In terms of atypical I-V characteristic (see FIG. 9, later) this corresponds toservoing around the maximum power point, more particularly moving alongthe characteristic curve in a direction of decreasing current andincreasing voltage (as in the just mentioned example), or increasingcurrent and decreasing voltage, towards the maximum power point.

In embodiments the power injection control block generates a template ofthe AC current injected into the mains. More particularly the templatecomprises a sinusoidal or half-sinusoidal voltage in phase with the gridmains and the amplitude of this template is adjusted dependent on themeasured DC link ripple voltage, more particularly dependent on whetherthis has previously gone up or down. Thus the amplitude of this templatesignal is responsive to the ripple voltage on the energy storagecapacitor/DC link. An error signal dependent on the difference betweenthe measured AC current injected into the grid mains and this templateis used to control the switching rate of the power injection controlblock. In embodiments the error signal is used to increase the switchingrate if the template magnitude is greater than the magnitude of thecurrent injected into the AC mains. In this way the current injected iscontrolled with the aim of maximising the energy storage capacitor/DClink ripple.

As previously mentioned, in some preferred implementations the rippleamplitude at the energy storage capacitor/DC link is used to effectivelymeasure power provided from the DC source (photovoltaic panel). Howeverin principle other techniques may be employed to measure, at the energystorage capacitor/DC link, the power provided from the DC power source.For example absent losses the power provided by the power source may beassumed to be given by the product of voltage on and current through theDC link providing an input to the DC-to-AC converter. Nonetheless,because preferred implementations of our power conditioning unit have aripple which is proportional to input power (assuming input and outputpower are substantially the same), measuring the ripple is anadvantageous technique for obtaining the desired power information.

Thus we describe a related method of maximum power point tracking (MPPT)in a power conditioning unit for delivering power from a dc power sourceto an ac mains power supply output, the power conditioning unitincluding an energy storage capacitor for storing energy from said dcpower source for delivering to said ac mains power supply output, themethod comprising: tracking a maximum power point of said dc powersource by controlling a dc-to-ac converter converting energy stored insaid energy storage capacitor to ac power for said ac mains power supplyinput, wherein said tracking comprises: sensing, at a circuit nodecoupled to said energy storage capacitor, a signal responsive to a levelof power drawn from said dc power source; and controlling said dc-to-acconverter to adjust an amplitude of an ac output to substantiallymaximise said sensed signal.

In embodiments the signal on the energy storage capacitors/DC link issensed and used to derive a control (template) signal having anamplitude dependent on the level of power drawn from the DC powersource, more particularly on a change in this sensed level of power.Then the switching rate of the DC-to-AC converter is controlled based ona difference between the sensed AC current and this control signal, moreparticularly increasing the switching rate of the output converter ifthe control signal (template) is greater than the sensed AC currentsignal, and vice versa.

Preferably a dc voltage amplification stage is included between the dcpower input and the ac mains output, and this stage has a substantiallyconstant amplification factor—that is it is not varied by a control loopto perform MPPT although, in embodiments, the constant amplificationfactor may be selectable, for example according to the operatingenvironment. In embodiments a voltage amplifier control block may beprovided, but not to provide a variable voltage amplification controlloop but instead to act effectively as a power switch to switch on andoff a path for power flow from the input to the dc-to-dc converter.Optionally, depending upon the implementation of the voltageamplification stage, the voltage amplifier control block may provide a(substantially constant duty cycle) pulse width modulation controlsignal to the dc voltage amplifier.

An arrangement of the type described above facilitates galvanicisolation between the dc input and ac mains power supply output sincethe MPPT tracking may be performed without any direct connection to thedc input for measuring voltage and/or current from the dc power source.

In an example implementation the dc-to-ac converter may comprise a buckstage converter or alternatively, for example, an “unfolding bridge” incombination with a pair of power switching devices and an outputinductor may be employed, as described in our U.S. Pat. No. 7,626,834(hereby incorporated by reference in its entirety). In embodiments thepower injection control block may be configured to sense a voltage onthe (dc link) energy storage capacitor, to scale this down, and tomultiply this by a sine wave (of appropriate phase) to create a templatesignal for comparison with a sensed signal derived from the grid mains,in order to control the output current of the dc-to-ac converter. Inembodiments no dc current sensing need be performed. In embodiments thepower conditioning unit may include an anti-islanding function, forexample as described in our co-pending U.S. application Ser. No.10/555,803 (WO2004/100348) (hereby incorporated by reference in itsentirety). In some preferred embodiments the energy storage capacitor isa non-electrolytic capacitor, for example a film, polyester, orpolypropylene capacitor; the capacitor may have a value of less than 50μF, 40 μF, 30 μF, 20 μF or 10 μF.

We also describe a method of maximum power point tracking (MPPT) in apower conditioning unit for delivering power from a dc power source toan ac mains power supply output, the power conditioning unit includingan energy storage capacitor for storing energy from said dc power sourcefor delivering to said ac mains power supply output, the methodcomprising: tracking a maximum power point of said dc power source bycontrolling a dc-to-ac converter converting energy stored in said energystorage capacitor to ac power for said ac mains power supply input,wherein said tracking is performed without measuring a dc voltage or dccurrent provided from said dc power source.

We further describe a method of maximum power point tracking (MPPT) in apower conditioning unit for delivering power from a dc power source toan ac mains power supply output, the power conditioning unit includingan energy storage capacitor for storing energy from said dc power sourcefor delivering to said ac mains power supply output, wherein, inoperation, a voltage on said energy storage capacitor has a sinusoidalvoltage component at twice a frequency of said ac mains, the methodcomprising: controlling an amplitude of an ac current provided to saidac mains power supply output such that an amount of power transferred tosaid ac mains power supply output is dependent on an amplitude of saidsinusoidal voltage component on said energy storage capacitor, whereinsaid controlling is performed by controlling a dc-to-ac converterconverting energy stored in said energy storage capacitor to ac powerfor said ac mains power supply input; and tracking a maximum power pointof said dc source by controlling said dc-to-ac converter.

Broadly in embodiments of such methods changing the fluctuatingsinusoidal component of voltage on the (dc link) energy storagecapacitor changes the voltage at the input from the dc power source andthe current (from the dc power source) is forced to follow the change involtage, in accordance with the current—voltage characteristic of the dcpower source. If power is drawn from the dc link and provided to the acmains output the dc voltage on the energy storage capacitor drops andthe dc input voltage drops concomitantly (and vice versa). Thus inembodiments of the method sensing (just) the voltage on the energystorage capacitor can be employed to control both current and voltage atthe input of the power conditioning unit.

These methods may be implemented using processor control code forcontrolling a processor to implement the method, the code being storedon a carrier such as non-volatile memory.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows an example power conditioning unit suitable forimplementation of an MPPT tracking system according to an embodiment ofthe present invention.

FIG. 2 shows details of the power conditioning unit suitable of FIG. 1.

FIG. 3 shows the DC capacitor voltage in the power conditioning unit ofFIG. 1.

FIG. 4 shows control block A in the power conditioning unit of FIG. 1.

FIG. 5 shows example characteristics of a photovoltaic panel array asknown in the art.

FIG. 6 shows control block B in the power conditioning unit of FIG. 1.

FIG. 7 shows details of examples of control blocks A and B for the powerconditioning unit of FIG. 1.

FIG. 8 shows output and input powers for the power conditioning unit ofFIG. 1.

FIG. 9 shows an example output current-voltage characteristic of aphotovoltaic panel indicating the location (X) of a maximum output powerpoint.

FIG. 10 shows a block diagram of an example dc input portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention.

FIG. 11 shows a block diagram of an example ac output portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention.

FIG. 12 shows a circuit diagram of an example dc input portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention.

FIG. 13 shows details of an ac output portion of a photovoltaic powerconditioning unit incorporating an MPPT tracking system according to anembodiment of the invention.

FIG. 14 shows the voltage on a DC link capacitor voltage in aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention, illustrating asinusoidal component of the voltage.

FIG. 15, shows an example control procedure for the power injectioncontrol block of a power conditioning unit with maximum power pointtracking according to an embodiment of the invention.

FIGS. 16 a and 16 b show first and second examples of a solarphotovoltaic power generation system architecture according toembodiments of the invention.

FIG. 17 shows a set of output current-voltage characteristic curves of aphotovoltaic panel indicating variation of the location (X) of themaximum output power operating point under varying conditions.

FIGS. 18 a and 18 b show, respectively, an example internal circuit of aPV panel, and a further, less preferred architecture for a solarphotovoltaic power generation system.

FIG. 19 shows a three-phase example of a solar photovoltaic powergeneration system architecture according to an embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Power Conditioning Units

We first describe examples of photovoltaic power conditioning units inthe context of which embodiments the MPPT (maximum power point tracking)techniques we describe may advantageously employed.

Thus we will first describe a method to control direct current energysources, in particular a method to control direct current energy sourcesthat utilise power electronics converters to condition the input powerinto alternating current electricity that is supplied to the mains. Suchpower electronics converter comprises of a plurality of conversionstages and one energy reservoir in the form of a capacitor. The methodpresented allows the utilisation of long-lifetime polyester orpolypropylene capacitors as opposed to short-lifetime electrolyticcapacitors. The method uses two control algorithms: one algorithmcontrols the power extracted from the energy source that is supplied tothe energy reservoir and another controls the transfer of power from thereservoir into the electricity mains.

In one arrangement there is provided a power conditioning unit fordelivering power from a dc power source to an ac mains power supplyoutput, the power conditioning unit comprising: a power conditioningunit input for receiving power from said dc power source; a powerconditioning unit output for delivering ac power; an energy storagecapacitor; a dc-to-dc converter having an input connection coupled tosaid power conditioning unit input and an output connection coupled tothe energy storage capacitor; and a dc-to-ac converter having an inputconnection coupled to said energy storage capacitor and an outputconnection coupled to said power conditioning unit output; wherein saidenergy storage capacitor is a non-electrolytic capacitor; and whereinsaid power conditioning unit comprises two control blocks, a first,power extraction control block to control said dc-to-dc converter tocontrol power extracted from said dc power source and provided to saidenergy storage capacitor, and a second, power injection control block tocontrol said dc-to-ac converter to control power injected into said acmains power supply from said energy storage capacitor; and wherein saidpower extraction control block has an input coupled to said powerconditioning unit input for receiving power from said dc power sourceand has an output to control said dc-to-dc converter and is configuredto regulate a voltage of said dc power source to control power extractedfrom said dc power source to said energy storage capacitor.

The ac mains power supply output may be connected to the utility grid,so that the power conditioning unit delivers power into the grid, or itmay be a standalone power supply output for supplying power toelectrical appliances.

The dc-to-ac converter may be configured to deliver a substantiallysinusoidal current or voltage to the ac mains power supply outputregardless of a voltage on the energy storage capacitor. This may beachieved by maintaining a current or voltage to the power supply outputsubstantially at a reference sinusoid current or voltage. This maycomprise controlling transistors in the dc-to-ac converter responsiveboth to a voltage or current from the energy storage capacitor and tothe current or voltage to the power supply output.

The energy storage capacitor may comprise a non-electrolytic capacitorsuch as a film-type capacitor (for example polyester or polypropylene).The value of the capacitance may be directly proportional to the maximumpower transfer capability, that is, the rated power of the apparatus.This value may be lower than that of the capacitor in a conventionalpower conditioning unit with the same power rating. For example, lessthan 20 microfarads, less than 15 microfarads, less than 10 microfarads,less than 5 microfarads or another size available for a non-electrolyticcapacitor.

We will also describe a dc-to-dc converter for delivering power from adc power source to a dc output, the converter being configured tomaintain a voltage on the dc power source substantially constant over arange of dc output voltages, the converter comprising an input forreceiving power from said dc power source, an output for delivering dcpower, at least one power device for transferring power from the inputto the output, a sensing circuit for sensing a voltage on said input,and a driver circuit for driving said at least one power deviceresponsive to said sensing to control said power transfer.

We will also describe an inverter for delivering power from a dc powersource to an ac output, the inverter being configured to maintain asubstantially sinusoidal output voltage or current over a range of dcpower source voltages, the inverter comprising an input for receivingpower from said dc power source, an output for delivering ac power, atleast one power device for transferring power from the input to theoutput, a low-pass filter coupled to said input, a sensing circuit forsensing an output from the low-pass filter and comparing with areference, and a driver circuit for driving said at least one powerdevice responsive to said sensing to control said power transfer.

We will also describe a power conditioning unit for delivering powerfrom a dc power source to an ac mains power supply output, wherein alink capacitor of the power conditioning unit connected in parallelbetween an output of a dc-to-dc converter of said power conditioningunit and an input of a dc-to-ac converter of said power conditioningunit is not an electrolytic capacitor.

We will also describe a method of controlling a power conditioning unitfor delivering power from a dc source into an ac electricity supply, thepower conditioning comprising: an input for connecting the dc powersource; an output for connecting the electricity supply; a first,dc-to-dc power conversion stage for voltage conditioning of the dc powersource; a second power conversion stage for power injection into the acelectricity supply; and a dc link energy storage capacitor for energybuffering from the dc power source to the electricity supply; whereinthe method comprises controlling said second power conversion stage tocontrol an amplitude of an ac current provided to said ac electricitysupply output such that an amount of power transferred to said ac mainspower supply output is dependent on a peak amplitude of a fluctuatingsinusoidal component of a dc voltage on said energy storage capacitor.

Thus an example power conditioning unit uses a system for controllingthe transfer of power from a dc energy source, such as a solar panel,fuel cell, dc wind turbine, etc, into the electricity mains supply, andin particular allows the replacement of short-lifetime energy reservoirsby long-lifetime polyester or polypropylene capacitors.

The energy control and MPPT techniques we describe can be used in anypower electronics converter device (1) as shown in FIG. 1. Thisapparatus (1) is made of three major elements: a power converter stage A(3), one reservoir capacitor C_(dc) (4), and one power converter stage B(5). The apparatus (1) has a plurality of inputs connected to a directcurrent (dc) source, such as a solar or photovoltaic panel array (2)comprising one or more dc sources connected in series and/or inparallel. The apparatus (1) is also connected to the electricity supply(6) so that the energy extracted from the dc source (1) is transferredinto the mains (6).

The power converter stage A (3) may be of different types: it can be astep-down converter where the voltage at the input is decreased usingsome power electronics topology; it can be a step-up converter where theinput voltage is amplified using a different type of power electronicscircuit; or it can do both amplify and attenuate the input voltage. Inaddition, it may provide electrical isolation by means of a transformeror a coupled inductor. In whatever case, the electrical conditioning ofthe input voltage should be such that the voltage across the capacitorC_(dc) (4) remains higher than the grid voltage (6) magnitude at alltimes. Also, this block contains one or more transistors, inductors, andcapacitors. The transistor(s) are driven through a pulse widthmodulation (PWM) generator. The PWM signal(s) have variable duty cycle,that is, the ON time is variable with respect to the period of thesignal. This variation of the duty cycle effectively controls the amountof power transferred across the power converter stage A (3).

The power converter stage B (5) injects current into the electricitysupply (6). Therefore, the topology utilises some means to control thecurrent flowing from the capacitor C_(dc) (4) into the mains (6). Thecircuit topology can be either a voltage source inverter or a currentsource inverter.

FIG. 2 shows an example of a power conditioning unit to which thecontrol system of FIG. 1 may be applied. In FIG. 2 Q1-Q4, D1-D4 and thetransformer form a voltage amplifier; Q9, D5, D6 and Lout performcurrent shaping; and Q5-Q6 constitute an “unfolding” stage. Control A (7in FIG. 1) may be connected to the control connections (e.g. gates orbases) of transistors in power converter stage A (21) to control thetransfer of power from the dc energy source (20). The input of thisstage is connected to the dc energy source and the output of this stageis connected to dc link capacitor 22. This capacitor stores energy fromthe dc energy source for delivery to the mains supply (24). Control Amay be configured to draw a substantially constant power from the dcenergy source regardless of the dc link voltage V_(dc) on C_(dc).

Control B (8 in FIG. 1) may be connected to the control connections oftransistors in power converter stage B (23) to control the transfer ofpower to the mains supply. The input of this stage is connected to thedc link capacitor and the output of this stage is connected to the mainssupply. Control B may be configured to inject a substantially sinusoidalcurrent into the mains supply regardless of the dc link voltage V_(dc)on C_(dc).

The capacitor C_(dc) (4) acts as an energy buffer from the input to theoutput. Energy is supplied into the capacitor via the power stage A (3)at the same time that energy is extracted from the capacitor via thepower stage B (5). The system provides a control method that balancesthe average energy transfer and allows a voltage fluctuation, resultingfrom the injection of ac power into the mains (6), superimposed to theaverage dc voltage of the capacitor C_(dc) (4), as shown in FIG. 3. Thefigure shows an average voltage of 475V and a 100 Hz fluctuation of peakamplitude of 30V. The peak amplitude depends on the amount of powerbeing transferred from the input (2 in FIG. 1) to the output (6). Thefrequency of the oscillation can be either 100 Hz or 120 Hz depending onthe line voltage frequency (50 Hz or 60 Hz respectively).

Two synchronised and independent control blocks control the system (1):a control block A (7) that directly controls the power stage A (3), anda control block B (8) that directly controls the power stage B (5).

Control block A (7) has the configuration shown in FIG. 4. It comprisesan adder (31), a negative proportional gain (32), a PWM generator (33),the system plant (34), and a feedback gain (35). This control blockregulates the voltage across the dc source (2). This voltage, v_(in), ismeasured and adjusted by gain k₁ (35). It is then subtracted to avoltage reference, V_(ref), using the adder (31). The error,(v_(ref)−k₁v_(in)), is then amplified by a factor of −k₂. The resultingsignal is negatively proportional to the error. Therefore, a positiveerror generates a decrement in the driving signal and conversely. Thisdriving signal is input to a PWM generator (33) that can be amicrocontroller, or a PWM integrated circuit. This block generatesdigital pulses that, in turn, drive the transistors of the power stage A(3) that is equivalent to the plant (34).

Controlling the dc source (2) voltage directly controls the power beingtransferred across power stage A (3) as is shown in FIG. 5 for aphotovoltaic panel array.

Control block B (8) has the configuration shown in FIG. 6. It comprisesan adder (41), a sample and hold (SH) with period T block (42), aproportional-derivative (PD) compensator (43), the system plant (44), alow-pass filter (LPF) feedback block (45). This control block regulatesthe average voltage across capacitor C_(dc) (4). Because the voltage,v_(dc), contains the sum of a constant voltage and a fluctuatingsinusoidal component, the signal is scaled and filtered using the LPFblock (45). This generates a constant voltage that is compared against areference, V_(dc) _(—) _(ref), using adder (41). The error is measuredevery T seconds using a Sample and Hold, SH, block (42). The resultingsampled error is forwarded to a PD compensator (43) that sets theamplitude of the current injected to the mains (6) via power stage B(5). The update of this current reference, I_(ref), amplitude is doneevery T seconds, which is the inverse of the line voltage frequency.Hence, it can take the values of 0.02 or 0.0167 seconds for a linefrequency of 50 or 60 Hz respectively. This is needed in order toprevent current injection distortion.

An implementation of control blocks A and B is shown in FIG. 7. Bothblocks operate independently but share a common microcontroller forsimplicity. The microcontroller performs the control strategy depictedin FIG. 6 for block B. In addition the microcontroller could incorporatesome means of maximum power point tracking control in case the inputsource is a photovoltaic panel in block A in order to generate areference input voltage used in FIG. 4. Consequently the input voltageand current and the dc-link voltage are fed into the microcontroller viaan arrangement of operational amplifiers or signal conditioning blocks.

The control shown in FIG. 4 for block A is implemented using analogueelectronics in the form of operational amplifiers and the phase-shiftPWM controller depicted in FIG. 7 (51). As mentioned before, the inputvoltage reference is obtained through the microcontroller via a digitalto analogue converter (DAC). The proportional error is obtained insidethe phase-shift PWM controller that, in turn, generates PWM signals forthe transistors of stage A (21).

Implementation of control B (52) includes a current transducer thatsenses the rectified output current. This signal is conditioned toappropriate voltage levels using operational amplifiers and is thencompared against a reference current. The reference current is generatedin the microcontroller by an algorithm shown in FIG. 6 and the resultingdigital word is sent to a DAC in order to get an analogue,instantaneous, current reference. Changes to the current magnitude aredone in a periodic basis (with period equal to the grid voltage period)in order to avoid current distortion. The result of the comparisonbetween the reference and the actual current is buffered through a Dflip-flop which, in turn, drives transistor Q9 in FIG. 2. TransistorsQ5-Q8 form a full-bridge that switches at line frequency using ananalogue circuit synchronised with the grid voltage. Transistors Q5 andQ8 are on during the positive half cycle of the grid voltage and Q6 andQ7 are on during the negative half cycle of the grid voltage.

FIG. 8 shows the output and input powers using the aforementionedcontrol. Clearly, the instantaneous power output is a sinusoidsuperimposed to an average positive value. In contrast, the input isconstant throughout the period of the line voltage. The power differencecreates an energy mismatch that is absorbed in capacitor C_(dc). Thiseffectively appears as a fluctuation across the capacitor, as is shownin FIG. 3.

MPPT (Maximum Power Point Tracking) Techniques

We will describe a method and system for tracking the maximum powerpoint of an energy generator and extracting maximum power from such agenerator when coupled to the load. In embodiments the method/systemcomprises two independent control blocks. The first block controls thevoltage amplification stage that interfaces with the energy generator.The energy generator is preferably a solar module. In embodiments thefirst control block does not function to regulate the amount of energyto be transmitted but functions only as a switch, either allowing energyflow or preventing any energy flow from the generator and through theamplification stage, regardless of the amount. The output of the voltageamplification stage is coupled to an energy reservoir capacitor. Energyflow is therefore dependent on the amount of “room” (the amount ofadditional energy which can be stored) in the reservoir capacitor. Thesecond control block is a feedback control loop that interfaces theenergy reservour capacitor to the coupled output load. The secondcontrol block regulates the amount of power to be injected into the loadby emptying the energy reservoir capacitor. The second control blockuses, in embodiments exclusively, the level of voltage fluctuations onthe energy reservoir (storage capacitor) to control the amount of powerbeing extracted from the energy generator and also the amount of powerbeing injected into the load. In embodiments no use of (measured)current values is made. Thus in embodiments the maximum power pointtracking uses two completely independent loops and uses exclusivelyvariations characteristic of the reservoir capacitor.

Some energy generators, such as solar photovoltaic cells, constitute anon-linear power characteristics profile such as one illustrated in FIG.9. In the figure maximum power is harvestable at the point labelled X,which exhibits maximum power point current Imp and voltage Vmp. It ispreferable that the operating point that yields most energy is attainedand maintained. The method we describe does not use the voltage andcurrent values measured at the output of the generator to performmaximum power point tracking. Instead the method measures the voltagefluctuations in the DC link and uses the measured values to track themaximum power point.

Referring to FIGS. 10 and 11, these show a block diagram of input 1002and output 1004 stages of an embodiment of a solar PV power conditioningsystem 1000 incorporating an MPPT control methodology for the dc inputside of the power conditioning unit according to an embodiment of theinvention. Thus FIG. 10 shows an energy generator 1010 such as one ormore PV panels feeding a voltage amplification stage 1012 with asubstantially constant amplification factor (which may be less than,equal to, or greater than unity depending, for example, on whether thedc input is from a single PV panel or a string of series connectedpanels). This in turn provides power to an energy reservoir 1014, inembodiments a storage capacitor coupled to a dc link between the input,voltage amplification stage and an output, voltage inversion stage.Control block A 1016 controls voltage amplification stage 1012, but inembodiments only to switch power from the energy generator on and offinto the energy reservoir. In embodiments control block A does notprovide a variable gain control and simply comprises a fixed frequencyoscillator. Voltage inversion stage 1018 has an input coupled to theenergy reservoir 1014 and provides an ac mains output to load 1020, forexample via a grid connection. Control Block B 1022 monitors the voltageon the dc link via sense connection 1022 a (but in embodiments does notsense the current on this link), and the current into and voltage on theload via sense connections 1022 b,c (in embodiments connection 1022 c iswithin the power conditioning unit), and provides gate drive outputsignals 1022 d for controlling the voltage inversion (“unfolding”) stage1018, more particularly for controlling the power drawn from the energyreservoir and provided into the load via the grid. The gate drivesignals 1022 d are sequenced to control the power converter switches ofthe power conversion stage 1018 (see also FIG. 2); this provides aconvenient technique for controlling the switching frequency of thisstage.

In FIG. 10, control block A functions as a power switch, allowing powerto flow from the energy generator to the voltage amplification stage (oreffectively switching the voltage amplification stage on/off or in/out).Control block A can also be set to turn off power from the energygenerator in the event of over-voltage and under-voltage conditions.

The voltage amplification stage can have a fixed amplification ratio ora selectable or multiplexable ratio such as may be provided by a tappedtransformer. The voltage amplification stage may comprise a half-bridge,a full bridge, a push-pull or a similar voltage inversion stage. Such aninversion stage may comprise semiconductor switching devices such asMOSFETs. The voltage inversion stage can be coupled to a transformer,whose amplification ratio results in a desired voltage in the DC linkreservoir capacitor, for example of order 400 volts. The output of thetransformer is coupled to a rectifier stage. An inductor may be includedbetween the rectifier bridge and the DC link reservoir capacitor.

Depending on the input voltage the voltage amplification stage 1012 mayprovide an amplification in the range ×5 to ×20, for example around ×12for a dc input voltage of ˜35 volts, giving a dc link voltage of around420 volts.

FIG. 12 shows a more detailed circuit diagram of an example input stage1002 implementing the control methodology we describe. The energygenerator may be a solar module or a group of solar modules. In thisexample the voltage amplification stage comprises a half-bridge, whichin turn is made up of two series switches (MOSFETs), Q1 and Q2, and twoseries capacitors C1 and C2, and the transformer TX1. A rectifier bridge1013 made up of diodes is coupled to the output of the transformer. Therectifier bridge is itself coupled to the DC link capacitor Cd via afilter inductor Ld. The control block in FIG. 12 produces a constantduty cycle PWM signal, and hence no modulation is implemented. In theevent that Cd is full, defined as the voltage across it being equal orlarger than the rectified output from transformer secondary, no powerflows into Cd even though Q1 and Q2 are switched on and offcontinuously. Hence control block A does not regulate the amount ofpower extracted from the generator.

FIG. 13 shows a more detailed circuit diagram of an example output stage1004 implementing the control methodology we describe. Referring to FIG.13, control block B measures the voltage fluctuations in the DC linkthat are used for regulation of the amount of power being harvested fromthe energy generator and therefore the amount of power injected into theload. A preferred load is the utility grid. In the case of the gridload, control B measures the peak and trough voltages on the DC linkcapacitor via a scaling circuit (the potential divider circuit of R3 andR4). The scaled values of the peak Vp and the trough Vt voltages areused to compute the amount of power flowing through the capacitor (asdescribed below). In embodiments the voltage sense connection to ControlBlock B is via a rectifier).

Energy Storage and DC Link Capacitance

Due to the AC nature of the power being transferred into the grid andthe current-voltage characteristic of the power being generated by thesolar module, energy storage is essential in a PV inverter if maximumpower is to be harvested from the solar module. In our preferred design,energy storage is delegated to the DC link reservoir capacitor. Theamount of power transferred into the grid is related to the energychange in the capacitor and therefore the voltage ripple on thecapacitor. One advantage of implementing energy storage on the DC linkis that a large ripple can be allowed on the capacitor. Equation 1illustrates the relationship between energy change, the capacitance andthe voltage on the capacitor:U _(R)=½C _(dc)(V _(P) ² −V _(T) ²)  (1)where V_(P) is the capacitor peak voltage and V_(T) is the capacitortrough voltage. The power transferred would be the energy change persecond. FIG. 14 illustrates the fluctuation(sinusoidal ripple) on the DClink capacitor.

Thus block B automatically achieves MPPT by regulating the amount ofinjected current with reference to (dependent on) the dc link voltagefluctuation.

However, the MPPT tracking technology we have described is notrestricted to operating in the context of a power conditioning unitwhich deliberately allows (and controls based on) a degree of ac rippleon the dc link. It may therefore be helpful to enlarge upon thedescription of the operation of embodiments of the technique.

Consider an input current and voltage I, V to the inverter provided by aphotovoltaic power source, a dc link current and voltage I_(d), V_(d),and a output current and voltage into grid mains of I_(grid), V_(grid).Since V_(grid) is approximately constant, the power injected into thegrid mains is proportional to I_(grid). Also, absent losses, the inputpower I.V=I_(d). V_(d). and thus I_(d). V_(d) determines the point onthe photovoltaic IV characteristic at which the system operates. Broadlyspeaking the system senses the ripple on V_(d) which, in embodiments,(as described above) is a measure of the power flowing through the dclink. More particularly the system controls the output “unfolding” stage(for example a buck stage converter) to maximise the level (amplitude)of this ripple component on the dc link voltage/energy storagecapacitor, and hence also to maximise the power injected into the acmains. (The skilled person will appreciate that V_(d) on its own doesnot provide a good measure of the power on the dc link).

In a preferred implementation the control block 1022 generates a halfsinusoidal template voltage (with an amplitude varying between zero and3.3 volts) in phase with the grid, for comparison with a (rectified)version of the sensed load current 1022 b. The sensed load voltage 1022c is used only to determine the phase of the ac mains. The amplitude ofthe template is adjusted dependent on the level of ripple sensed on theenergy storage capacitor/dc link (via line 1022 a). If the templateamplitude is greater than the amplitude of the sensed grid current theswitching frequency is increased to inject more power into the grid, andvice versa. Thus, broadly speaking, the amplitude of the template isadjusted dependent on the dc link ripple and the output current iscontrolled to match the template amplitude.

Referring now to FIG. 15, this shows an example control procedure forcontrol block B 1022. FIG. 15 is an example; the skilled person willappreciate that many variations are possible.

Presuming that the procedure begins at start-up of the inverter, theprocedure first initialises the amplitude of the template signal to anarbitrary, relatively low value, for example 0.5 volts on the previous0.-3.3 volts scale (step S1500). Referring again to FIG. 9, at thispoint the output voltage from the photovoltaic panel is at a maximum andthe output current is at substantially zero; the level of ripple on thedc link is also substantially zero.

The procedure determines the phase of the ac grid mains voltage (S1502)and synchronises the half-sinusoidal template to the grid. The procedurethen senses the grid current (S1504), for example by sensing the voltageacross a current sense resistor; at start-up this will be approximatelyzero. The procedure then determines an error value E from the differencebetween the template and the sensed grid current (S1506), which atstart-up (continuing the previous example) will be 0.5. The procedurethen determines a switching rate for the voltage inversion stage 1018dependent upon this error, in one example algorithm increasing theswitching rate if E is positive and decreasing the rate if E isnegative. Thus in the present example, at start-up the templateamplitude is greater than that of the sensed grid current so theswitching rate is increased. This increases the current (and hencepower) injected into the ac grid mains, so that the ripple voltage onthe dc link also increases.

At step S1510 the procedure measures the ripple voltage on the dc linkand, at step S1512, adjusts the template amplitude dependent on thismeasurement, more particularly increasing the amplitude if the ripplevoltage increased, and vice versa. The procedure then loops back to stepS1504 to once again sense the current being injected into the ac mains.Thus, for example, if the error is positive the template amplitudeincreases so that it is once again greater than the amplitude of thesensed current injected into the grid, and thus the switching rate ofthe voltage inversion stage is once again increased. However if theprevious change decreased the measured ripple voltage (which senses thepower drawn from the photovoltaic panel), then the template amplitude,and hence switching rate of the voltage inversion stage, is alsodecreased. In this way the control technique operates to control theoutput voltage inversion stage such that the photovoltaic panel ismaintained at substantially its maximum output power point.

We have thus described a power conditioning unit with MPPT for aphotovoltaic panel in which a power injection control block has a senseinput coupled to an energy storage capacitor on a dc link and controls adc-to-ac converter to control the injected mains power. The powerinjection control block tracks the maximum power point by measuring asignal on the dc link which depends on the power drawn from the dc powersource, and thus there is no need to measure the dc voltage and currentfrom the PV panel. In embodiments the signal is a ripple voltage leveland the power injection control block controls an amplitude of an accurrent output such that an amount of power transferred to the gridmains is dependent on an amplitude of a sinusoidal voltage component onthe energy storage capacitor.

The MPPT tracking techniques are preferably implemented in an inverterof the general type described above. However the techniques may also beused with other types of inverter, for example a ‘four-switch’ inverteras described in our patent U.S. Pat. No. 7,626,834 (in particular ifthis is provided with a half or full bridge dc boost stage (with atransformer) at the front end).

Solar Photovoltaic System Architectures

We will now describe an architecture of a parallel-connected solarphotovoltaic power generation system according to an embodiment of theinvention.

Referring to FIGS. 16 a and 16 b these show respective embodiments 1600,1650 of a solar photovoltaic power generation system according to theinvention. In each case two photovoltaic panels 1602, 1604 are connectedin parallel to the dc input of a solar inverter 1000 including an MPPTtracking system, as previously described. In FIG. 16 a the parallelconnections are external to the inverter; in FIG. 16 b they areinternal.

In embodiments the inverter 1000 is a microinverter, for example with amaximum power rating of less than 2 KW, 1 KW or 600 watts. Themicroinverter is upgraded to handle the increased power available fromtwo (or more) panels by increasing the value of the energy storagecapacitor (albeit in embodiments this still remains relatively small,for example less that 50 μF). Referring to FIG. 2, in embodiments achoke may be included between the output of the dc-to-dc converter(power converter stage A) and the energy storage capacitor in the dclink, to snub the magnetizing current out of the transformer. Thiscomponent may also need to be up-rated, for example to around 2 amps.

In the example architectures of FIG. 16 preferably the panels are allsubstantially matched, that is of substantially the same size/type/powerrating. This helps to achieve good MPPT tracking performance. Althoughtwo parallel connected panels are shown, more may be added.

Referring now to FIG. 17 this illustrates how the previously describedMPPT tracking operates with two parallel-connected panels. Curve 1700 ofFIG. 17 shows an I-V curve for each of a pair of parallel-connectedpanels, and curve 1702 illustrates the shift when one panel isshaded/dirty. The maximum power points are indicated by “X”, and it canbe seen that when at the same voltage both panels can be substantiallyat the maximum power point even when one is shaded. This is because theshift between the curves is relatively small even when one panel isshaded—for there to be a substantial change in the maximum power point,as shown for example by curve 1704, there would need to be extremeshading (so that, for example, a bypass diode conducts) and/or failureof part of a panel, for example by failure of a diode.

To facilitate understanding of the operation of embodiments, FIG. 18 ashows an example internal construction of a photovoltaic panel 1800,here comprising three strings of, for example 24, diodes 1802, 1804,1806, each string being provided with a respective bypass diode 1802 a,1804 a, 1806 a. The p-n junctions of the diodes in the strings eachgenerate a voltage which may typically be of order 0.5 volts.

FIG. 18 b shows a power generation system 1850 with parallel connectedseries-coupled panels 1602, 1606 and 1604, 1608. This works, but unlikewith directly parallel connected panels if one panel in the arrangementof FIG. 18 b is shaded this generates a voltage offset because the panelis series (as well as parallel) connected. This reduces theeffectiveness of the MPPT when the panels are unevenly illuminated. Thusin preferred embodiments the panels are directly parallel connected—thatis without intermediate series-connected panels. (The skilled personwill appreciate that directly parallel connected panels may be connectedeither internally or externally to the inverter when making the parallelconnections). One potential advantage of the arrangement of FIG. 18 b,however, is that it can be employed to remove the need for an inputdc-to-dc converter to increase the dc input voltage, thus potentiallyavoiding the need for a transformer.

Referring again to FIG. 17, the I-V curves shown roll off withincreasing temperature—that is as temperature increases the voltageoutput of a PV panel decreases, as does the power output. For thisreason in some preferred embodiments the panels are arranged such that,in operation, they are approximately or substantially matched intemperature. This helps to avoid a significant voltage skew between thepanels (when measured at the panels), which could otherwise affect theMPPT tracking: broadly speaking in embodiments shading has little effecton the MPPT tracking until a bypass diode conducts but a significantdifference in temperature between the two panels could moresignificantly affect the MPPT tracking. The efficiency of an arrangementof the type shown in FIG. 16 a has been compared with an arrangement inwhich a separate inverter 1000 is provided for each PV panel and thedifference has been found to be low, of order 1 percent. However thearchitecture of FIG. 16 a provides a substantial saving in cost per wattand, potentially, also increased reliability.

Although some preferred embodiments of the invention employ MPPTtracking as described above, advantages are still potentially availablefrom a parallel-connected panel architecture where theparallel-connected panels are connected to a common microinverter with adifferent form of MPPT. This is in particular because scaling of thepower rating of the microinverter can be achieved primarily byincreasing the storage on the dc link, rather than by any substantialchange to the other components. Thus a microinverter, for example of thetype shown in FIG. 2, may be employed with MPPT tracking at the frontend and nonetheless still provide some useful advantages in terms ofcost-per-watt savings.

The techniques we have described are also applicable to the generationof a three phase ac grid mains power supply, for example using the PVpower generation system 1900 of FIG. 19. In this arrangement theinverter 1000′ is a modified version of inverter 1000, with threedc-to-ac converters 1018 a,b,c, one for each phase. The value of theenergy storage capacitor 1014 may be reduced since the requirements forenergy storage are less (because at any time at least one phase isalways exporting power). The previously described techniques for sensingthe ripple on the energy storage capacitor and using the peak (i.e. halfpeak-to-peak) value of the ripple to control the power injected into theac mains can still be employed, although the amplitude of the ripplevoltage is reduced.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the scope of the claims appended hereto.

What we claim is:
 1. A photovoltaic power generation system, the systemcomprising: at least two photovoltaic panels each having a dc poweroutput; a power conditioning unit having a dc power input and an acmains power supply output for delivering ac power to an ac mains powersupply; wherein said dc power outputs of said at least two photovoltaicpanels are connected in parallel with one another to said dc power inputof said power conditioning unit; wherein said power conditioning unitcomprises: a dc-to-dc converter having an input coupled to said dc powerinput and having an output coupled to a dc link of said powerconditioning unit; a dc-to-ac converter having an input coupled to saiddc link and having an output coupled to said ac mains power supplyoutput; and an energy storage capacitor coupled to said dc link; whereinsaid power conditioning unit is configured to perform maximum powerpoint tracking (MPPT) responsive to a level of power flowing into saiddc power input, wherein said level of power flowing into said dc powerinput is sensed at said dc link; wherein said power conditioning unitcomprises an MPPT controller coupled to control said dc-to-ac converterto perform said maximum power point tracking (MPPT); wherein saidcontroller has a sense input coupled to said energy storage capacitor tosense at said energy storage capacitor a signal responsive to the levelof power flowing into said dc power input of said power conditioningunit, and wherein said controller is configured to perform said MPPT bycontrolling said dc-to-ac converter to control power injected into saidac mains power supply responsive to said level of power flowing intosaid dc power input of said power conditioning unit sensed at said dclink and wherein said ac mains power supply output is a three phaseoutput, and wherein said power conditioning unit comprises one saiddc-to-ac converter per phase, each coupled to said dc link.
 2. Aphotovoltaic power generation system as claimed in claim 1 wherein avoltage on said energy storage capacitor has a sinusoidal voltagecomponent at twice a frequency of said ac mains power supply, whereinsaid sense input of said controller senses a value of said sinusoidalvoltage component, and wherein said controller is configured to controlan amplitude of an ac current provided to said ac mains power supplyoutput such that an amount of power transferred to said ac mains powersupply output is dependent on an amplitude of said sinusoidal voltagecomponent on said energy storage capacitor.
 3. A photovoltaic powergeneration system as claimed in claim 2 wherein said controller isconfigured to control said amplitude of said ac current provided to saidac mains power supply output to substantially maximize said value ofsaid sinusoidal voltage component.
 4. A photovoltaic power generationsystem as claimed in claim 1 wherein said controller is configured tocontrol said dc-to-ac converter to track a maximum power point of saiddc power outputs without directly measuring a dc voltage or dc currentprovided from said dc power outputs.
 5. A photovoltaic power generationsystem as claimed in claim 1 wherein said power conditioning unit has asingle, common MPPT control system for said at least two photovoltaicpanels.
 6. A photovoltaic power generation system as claimed in claim 1wherein said energy storage capacitor is a non-electrolytic capacitor.7. A photovoltaic power generation system as claimed in claim 1 whereinsaid power conditioning unit comprises a single said dc-to-ac convertercoupled to said parallel-connected photovoltaic panels.
 8. Aphotovoltaic power generation system as claimed in claim 1 whereinrespective positive and negative terminals of said photovoltaic panelsare connected to one another.
 9. A photovoltaic power generation systemas claimed in claim 1 wherein said photovoltaic panels comprise parallelseries-connected panels, wherein said photovoltaic panels are connectedin parallel to one another via one or more intermediate respectiveseries-connected panels.
 10. A photovoltaic power generation system, thesystem comprising: a first plurality of photovoltaic panels connected inseries with each other having a first positive connection and a firstnegative connection at either end of the first plurality of photovoltaicpanels connected in series; a second plurality of photovoltaic panelsconnected in series with each other having a second positive connectionand a second negative connection at either end of the second pluralityof photovoltaic panels connected in series; a power conditioning unithaving a dc power input and an ac mains power supply output fordelivering ac power to an ac mains power supply; wherein the dc powerinput comprises a positive connection and a negative connection; whereinthe first plurality of photovoltaic panels and the second plurality ofphotovoltaic panels are connected in parallel with one another bycoupling the first and second positive connections together and thefirst and second negative connections together, wherein the first andsecond positive connections are coupled to the positive connection ofsaid dc power input of said power conditioning unit and the first andsecond negative connections are coupled to the negative connection ofsaid dc power input of said power conditioning unit; wherein said powerconditioning unit comprises a dc-to-ac converter, and a dc link coupledto said dc power input of said power conditioning unit and an input ofsaid dc-to-ac converter, wherein said power conditioning unit does notinclude a voltage-increasing dc-to-dc converter between said dc powerinput and said dc link, wherein said dc-to-ac converter has an outputcoupled to said ac mains power supply output, and wherein said powerconditioning unit further comprises an energy storage capacitor coupledto said dc link; wherein said energy storage capacitor is anon-electrolytic capacitor; and wherein said power conditioning unit isconfigured to perform maximum power point tracking (MPPT) responsive toa level of power flowing into said dc power input; and a controllerconfigured to control an amplitude of an ac current provided to said acmains power supply output such that an amount of power transferred tosaid ac mains power supply output is dependent on a peak amplitude of afluctuating sinusoidal component of a dc voltage on said energy storagecapacitor, wherein said controller has a sense input coupled to saidenergy storage capacitor to sense at said energy storage capacitor asignal responsive to the level of power flowing into said dc power inputof said power conditioning unit, and a control output to control saiddc-to-ac converter such that the amplitude of the ac current provided tosaid ac mains power supply output is dependent on an amplitude of saidsinusoidal voltage component on said energy storage capacitor, andwherein said controller is configured to perform said MPPT bycontrolling said dc-to-ac converter.
 11. A photovoltaic power generationsystem as claimed in claim 10 configured to control the amplitude of theac current provided to said ac mains power supply output such that theamount of power transferred to said ac mains power supply output isdependent on the peak amplitude of the fluctuating sinusoidal componentof the dc voltage on said energy storage capacitor to control powerinjected into said ac mains power supply responsive to said level ofpower flowing into said dc power input of said power conditioning unitsensed at said dc link.
 12. A photovoltaic power generation system asclaimed in claim 10 wherein said ac mains power supply output is a threephase output, and wherein said power conditioning unit comprises onesaid dc-to-ac converter per phase each coupled to a common said dc link.